1. Field of the Invention
The present invention relates generally to opto-electronic converters, image reading devices, and image forming apparatuses.
2. Description of the Related Art
In recent years, CMOS (complementary metal oxide semiconductor) image sensors are increasingly used as image sensors in place of CCDs (charge coupled devices). This is because a dedicated process is required to manufacture pixels of a CCD and a plurality of power supply voltages is required to use the CCD. Further, because the CCD requires that a plurality of peripheral ICs (integrated circuits) be operated in coordination with the CCD, system complexity considerably increases. However, CMOS image sensors overcome these disadvantages.
CMOS image sensors can be manufactured using a manufacturing process similar to that for manufacturing general CMOS integrated circuits. Furthermore, a CMOS image sensor can be driven with a single power supply. Furthermore, a CMOS image sensor can be formed on a same chip together with analog circuitry and logic circuitry using a CMOS manufacturing process. Thus, CMOS image sensors provide considerable advantages including reduction in the number of peripheral ICs and allow reduction power consumption and size. For these reasons, CMOS image sensors come into wide use in mobile phones and digital cameras.
Mainstream CMOS image sensors have a column-based architecture including an FD (floating diffusion) amplifier for each pixel and an ADC (analog-to-digital converter) for each column of a pixel array, and configured to select one row from the pixel array, read out the row simultaneously in the direction of the column, and output pixel signals as digital signals.
A known type of CMOS linear sensors includes pixels of R (red), G (green), and B (blue), arranging in the main-scanning direction, and a processing circuit for processing three pixels R, G, and B as one column, and processing a signal output from each pixel in the order of R, G, and B.
Japanese Laid-open Patent Application No. 2010-259109 discloses a semiconductor device including a signal obtaining unit, amplifier circuits, and unit-signal detecting units. The signal obtaining unit includes unit elements arranged in a two-dimensional matrix of horizontal rows and vertical columns. Each of the unit elements includes a unit-signal generator configured to generate a unit signal corresponding to a signal charge generated by a charge generator. Each of the amplifier circuits is configured to amplify the unit signals read out from the unit elements by a gain applied to the amplifier circuit. Each of the unit-signal detecting units is configured to detect magnitude of the unit signals of one of vertical columns output from the signal obtaining unit and not yet, input to the amplifier circuit. Gains, each for a single column, to be applied to the amplifier circuits are individually determined based on detection results output from the unit-signal detecting units. Each of the amplifier circuits is operated using a corresponding one of the thus-determined gains.
However, in an image sensor where each column includes three pixels R, G, and B, if outputs of the R, G, and B pixels are amplified by a same PGA (programmable gain amplifier) and thereafter A/D-converted, signal levels of the outputs can vary from one color to another due to spectral characteristics of color filters of the pixels, a light source, and the like. This variation can disadvantageously narrow A/D-conversion dynamic range of a color of low output level. In that case, because the number of bits of conversion of an A/D converter is not effectively utilized, the gradation can deteriorate.
Even if a gain of the PGA is switched to a gain appropriate for a color of an image signal being output, influence of charge injection (injected charge) that occurs at the gain switching causes the potential across a capacitor that determines the gain to fluctuate. Accordingly, the need of waiting for the fluctuation to converge before amplifying the image signal arises, which can be a bottleneck for high-speed image reading.
In view of the above, there is a need for an opto-electronic converter, an image reading device, and an image forming apparatus capable of, even if levels of signals output and opto-electronically converted by a plurality of light-receiving elements included in a pixel group vary from one color to another of received light, adjusting the signal levels to appropriate levels on a per-color basis.